1. Field of the Invention
The present invention relates to an image forming apparatus capable of forming an image by using a plurality of laser beams, an information processing apparatus capable of communicating with the image forming apparatus, an information processing method, and a computer-readable storage medium.
2. Description of the Related Art
Conventionally, an image forming apparatus which forms an image by scanning and exposing a photosensitive member, which is provided as an image bearing member, to a laser beam, such as a laser printer and a laser copier, is known. In recent years, digital and color image forming apparatuses have become popular.
In forming a color image by using such image forming apparatuses, an original image corresponding to each of four colors (i.e., cyan (C), magenta (M), yellow (Y), and black (K)) is sequentially formed. Ultimately, the four original images are superimposed to form one color image. However, productivity of an image-forming operation of these types of image forming apparatuses is lower than a conventional image forming apparatus for forming black-and-white images.
As a solution to this problem, a tandem-type image forming apparatus is conventionally used. The tandem-type image forming apparatus can simultaneously form an original image corresponding to each of the C, M, Y, and K colors, and includes a plurality of photosensitive members. The photosensitive member corresponding to each of the four colors is exposed to a laser beam which is emitted from an exposure unit according to an image data signal separated into each of the four colors. Then the image formed on each of the photosensitive members is developed, and an original image for each of the four colors will be formed. Finally, by superimposing the original image of each of the four colors on a same transfer medium, a color image is formed on the transfer medium. In this way, productivity of the tandem-type image forming apparatus concerning the image forming operation is significantly increased.
Next, an example of a configuration of a scanning exposure unit that emits laser beams to scan and expose each of the photosensitive members will be described. The scanning exposure unit is included in the above-described tandem-type image forming apparatus.
FIG. 1 illustrates an image forming apparatus 100 including scanning exposure units 102C, 102M, 102Y, and 102K each of which deflects a laser beam emitted from a laser light source 103 using a polygonal mirror 104. The scanning exposure units 102C, 102M, 102Y, and 102K respectively corresponds to the four colors C, M, Y, and K and are arranged independently. In the image forming apparatus 100, each of the scanning exposure units 102C, 102M, 102Y, and 102K includes the polygonal mirror 104 which is driven by a motor (not shown). The polygonal mirror 104 deflects the laser beam and scans a corresponding photosensitive member 105 to expose a monochromatic image of each of the C, M, Y, and K colors thereon.
The monochromatic image which is exposed on the photosensitive member 105 corresponding to each of the four colors is developed by a respective developer 106, and then transferred onto a respective transfer unit 107 by a transfer belt 108. The transfer belt 108 is a transfer member common to the four colors. A fixing unit 109 is provided on the most rear-end side of the transfer belt 108. At the fixing unit 109, the monochromatic image of each of the four colors is sequentially superimposed on a recording medium 101, and one color image is formed thereby.
However, in the tandem-type image forming apparatus, positional misregistration due to, for example, variation of optical characteristic of the laser beam of each of the four colors emitted from the respective scanning exposure units may occur when the monochromatic image of each color is superimposed. Such positional misregistration may deteriorate a quality of the formed image. Thus, it is necessary to perform appropriate position adjustment control of each monochromatic image. Japanese Patent Application Laid-Open No. 05-083485 discusses a method for appropriately adjusting a position of monochromatic images, which can reduce color misregistration of each monochromatic image in a sub scanning direction to half a main scanning line interval or smaller.
When appropriate position adjustment is performed for original images to form a high-quality color image, it is necessary to appropriately set a predetermined parameter. Typical parameters that need to be set are as follows:
A write start position of a scanning line in a main scanning direction (hereinafter referred to as a “left registration”)
A write start position of a scanning line in a sub scanning direction (hereinafter simply referred to as a “scanning line write start position”)
A write end position or a print width of a scanning line in the main scanning direction (hereinafter referred to as “magnification”)
Curvature of a scanning line (hereinafter referred to as a “scanning line curvature”)
An inclination of a scanning line (hereinafter referred to as a “scanning line inclination”)
Next, position adjustment control of each monochromatic image performed in the tandem-type image forming apparatus will be described.
When a position of each monochromatic image is adjusted, it is necessary to correct the above described left registration, scanning line write start position, magnification, scanning line curvature, and scanning line inclination, and set appropriate values for them. Here, the correction of the scanning line curvature will be described as an example.
The scanning line curvature is related to an optical system of the scanning exposure unit. For example, the scanning line curvature occurs when an incident angle of the light incident on the polygonal mirror mounted on a polygonal mirror drive motor is not 90-degrees with respect to a reflection surface of the polygonal mirror. The polygonal mirror is a deflection unit of a scanning optical system in the scanning exposure unit. The scanning line curvature occurs since an optical path length to the polygonal mirror is changed depending on a rotation angle of the polygonal mirror at the time of scanning with the laser beam. When the optical path length is changed, a reflection position of the light on the reflection surface is also changed.
In a case of a plane mirror that simply reflects light on its reflection surface, the scanning line curvature may occur if the reflection surface is curved in the scanning direction since the optical path length is changed as is with the case described above. Since the scanning line curvature occurs due to an alignment of the optical system, as described above, the scanning line curvature occurs constantly. If a scanning line curvature is generated in a color printer that forms a full-color image by superimposing images formed by scanning lines of each color, color misregistration will be generated and a high-quality color image cannot be formed.
As a method for correcting the scanning line curvature, there is a conventional method, which corrects the scanning line on the photosensitive member by mechanically deforming the plane mirror of the scanning exposure unit and changing a curve of the scanning line opposite to the curvature of the scanning line. A scanning line of a scanning exposure unit using a cylindrical mirror can be corrected by deforming the cylindrical mirror, as well as the plane mirror.
There are methods to electrically correct the scanning line curvature. One of such methods discussed in Japanese patent No. 3202709 changes writing timing of image data depending on a scanning line curvature. Another method corrects the scanning line curvature by corresponding image data to be printed to the scanning line curvature and changing arrangement of the image data per pixel in advance on image memory. Additionally, there is a method to minimize the scanning line curvature by changing a light quantity of the laser so as to form, on the photosensitive member, a latent image whose pixel position is intentionally moved in the sub scanning direction.
When the position of each monochromatic image is adjusted, the above described left registration, scanning line write start position, magnification, scanning line curvature, and scanning line inclination are corrected and set to appropriate values.
However, the write start position of the scanning line in the sub scanning direction can be corrected only in units of one line in the sub scanning direction according to characteristics of the image forming apparatus that uses laser. One line is an interval of the main scanning lines. Thus, the positional misregistration of each color image may be up to half the interval of the main scanning lines in the sub scanning direction. Thus, as illustrated in FIG. 2, as to an amount of color misregistration between two colors, misregistration of up to one line in the sub scanning direction may occur.
If the color misregistration of the monochromatic images is as large as one line, a true color cannot be realized and the color image becomes low in quality.